A system that, for an input consisting of a number of signals, produces an output that is not the simple sum of the system’s response to each input considered separately. For example, if the input waveform x1[t] to a system produces the output y1[t] and the input x2[t] produces the output y2[t], then the system is nonlinear if the output is not precisely {y1[t] + y2[t]} when the input applied is {x1[t] + x2[t]}. A consequence of this is that we cannot determine the behaviour of the whole system by looking just at the effect of each individual input in turn. See also linear system; piece-wise linear function.